1. Field of the Invention
This invention relates to a magnetic tape transport, and in particular, to control of the tape velocity and tension in a capstanless magnetic tape transport.
2. Description Relative to the Prior Art
It is well known in the magnetic recording art that tape velocity and tape tension are important parameters in the design of a magnetic tape transport. Constant tape velocity is essential in preservation of timing relations and frequency content in the recorded data. The importance of tension control arises from two conflicting considerations in transporting a magnetic tape across a magnetic record or playback head. First, a high enough tension must be maintained to insure intimate contact between the tape recording surface and the magnetic head to prevent the loss of signal due to spacing between the magnetic surface of the tape and the head. This spacing loss becomes increasingly severe with decrease in recorded wavelength, so the modern trend towards shorter and shorter recorded wavelengths requires extremely intimate head to tape contact. This may be accomplished by an adequate tape tension having a component forcing the tape into contact with the surface of the head. On the other hand, magnetic tape is invariably abrasive, and a high value of tape tension may result in unacceptable head wear as the tape is repeatedly shuttled over the head. The tape tension, therefore, must be established and maintained at a compromise value which satisfies both requirements. Unlike the requirement for constant tape velocity, there is latitude in one direction or the other in setting the tension to effect this compromise, and a variation of 10 to 15 percent from an optimal tension value is generally acceptable.
In the prior art transport utilizing a capstan for tape speed control, a variety of methods have been employed to provide the required tape tension. One means employs vacuum columns for the tape entering and leaving the capstan region. Each vacuum column is maintained at a reduced pressure such that the atmospheric pressure on the tape travelling through the column results in a force on the tape which sets the tension at a predetermined value. Another method employed in transports of the prior art uses rotatable spring loaded arms over which the tape passes. The arms are servo controlled to maintain a position such that the tension in the tape tending to rotate an arm in one direction is just balanced by the force exerted by the spring for rotation in the opposite direction.
The capstanless transport has also been disclosed in the prior art, where the velocity and the tension of the tape are set by control of the currents in the supply and take up motors. U.S. Pat. No. 4,256,996 issued in the names of Brooks et al discloses such a capstanless transport using an approximate calculated speed reference for controlling the tape velocity, and utilizing the specific torque-speed characteristics of the supply motor for control of the tension.
U.S. Pat. No. 4,400,745 in the name of Shu utilizes an algorithm that the tape tension in a velocity controlled capstanless transport may be maintained at a substantially uniform value by keeping the sum of the currents of the take up motor and the supply motor equal to a constant. If, for example, an increase in bearing friction in one of the reeling motors causes a decrease in tape velocity, the velocity servo will demand increased torque from the velocity servo controlled motor to restore the tape velocity. This in turn, requires an increase in the velocity servo motor current. As the tape accelerates, if the hold back torque, i.e. the current in the supply motor, is not correspondingly reduced, the tape tension will increase. This interaction of the motor currents acts to keep the tape tension constant. U.S. Pat. No. 4,400,745 implements this algorithm by incorporating two closed loop servos in a capstanless tape transport. One closed loop maintains the velocity of the tape equal to a preset value, and the second closed loop maintains the sum of the currents in the two motors equal to a pre-set constant current value. It will be noted that these two closed feedback loops are tightly coupled through the tape under tension, complicating the stabilization of the loops as the tape pack radius decreases on one reel while it is increasing on the other reel.